1. Field of the Invention
The present invention relates to a small-sized motor for use in, for example, power tools and electric equipment for vehicles such as door locks and seat adjusters, and more particularly to a structure of a magnet for stator magnetic poles attached to the inner circumferential surface of a motor casing.
2. Description of the Related Art
FIG. 11 is a cross sectional view of a conventional small-sized motor as shown in Japanese Patent Application Laid-Open (kokai) No. H10-201206. This motor includes a rotor having three magnetic poles; a yoke disposed to surround the rotor; and two stator pole magnets attached to the inner surface of the yoke and each having an arcuate (C-shaped) cross section. Each of the magnets has such a shape that the gap between the magnet and the outer circumferential surface of the rotor gradually increases from a central portion to opposite end portions along the circumferential direction of the rotor.
Moreover, in each of the illustrated magnets, the back surfaces (outer surfaces) of the opposite end portions are cut so that cut portions are formed there. By means of changing the shape of these cut portions, magnetic flux density at the opposite end portions of each magnet can be adjusted.
In general, each stator pole magnet is formed such that the radial distance between the magnet and the outer circumferential surfaces of the rotor poles gradually increases from its circumferential center toward the opposite end portions so as to gradually weaken the magnetic field. This configuration makes it possible to avoid abrupt changes in the magnetic field, which would otherwise occur upon rotation of the rotor. Thus, generation of cogging torque can be mitigated.
However, since the illustrated motor requires an operation of assembling a plurality of (two) C-shaped magnets into the yoke, particularly when four or more magnets are assembled into the yoke, their positioning and fixing are difficult. That is, the structure of the illustrated motor is disadvantageous in terms of production cost and quality. Further, in order to reduce the magnetic flux from the central portion toward the opposite ends thereof, the illustrated magnet is formed in such a manner as to gradually increase the gap between the magnet and the outer circumferential surface of the rotor from the central portion toward the opposite ends thereof. However, such a shape reduces the magnetic flux generated by use of an excessively large magnet by widening the gap, and therefore is not efficient.